1. Field of the Invention
The present invention relates to a method of improving registration accuracy and to a method of forming patterns on a substrate using the method. More specifically, the present invention relates to a method of improving registration accuracy in a step of exposure during manufacturing of a semiconductor device, and to a method of forming patterns on a substrate using the method.
2. Description of the Background Art
Conventionally, measurement of positional offset of a registration accuracy measurement mark has been utilized as a method of measuring an amount of offset in registration during the steps of manufacturing a semiconductor device. In the measurement of offset of the registration accuracy measurement mark, first, a registration accuracy measurement mark is formed, and a position of a step of the measurement mark is found. Based on the position of the step, the position of the measurement mark is found. Further, based on the position of the measurement mark, an amount of offset in registration is measured. As to a specific method of finding the position of the step, a method has been known in which light beams are directed to the step and intensity of reflected light beams is found. In this method of finding the position of the step in accordance with the intensity of reflected light beams, the position of the step is detected utilizing the phenomenon that light beams are reflected irregularly at the step so that intensity of the reflected light beams is lower at the step than other portions. That is, a portion where intensity of reflected light beams is lower is found.
The conventional method of improving registration accuracy will be described with reference to FIGS. 10 to 13.
In the conventional method of improving registration accuracy, referring to FIG. 10, positions of registration accuracy measurement marks are measured at measurement points on selected ones of a plurality of chips 102 formed on a wafer 101. In each measurement point, first and second registration accuracy measurement marks 201 and 202 are formed simultaneously with patterning of interconnections and the like. First registration accuracy measurement mark 201 is formed on the substrate, and second registration accuracy measurement mark 202 is formed on first registration accuracy measurement mark 201. FIGS. 11 and 12 are plan view and cross sectional view, respectively, of the first and second registration accuracy measurement marks 201 and 202 of this example.
At the time of inspection, referring to FIG. 12, light beams 204 are directed to first and second registration accuracy measurement marks 201 and 202. Of the light beams 204, a beam incident on an upper surface of second registration accuracy measurement mark 202 will be referred to as incident light beam 204a, and a beam reflected at the upper surface of second registration accuracy measurement mark 202 will be referred to as reflected light beam 204b. A beam incident on an upper surface of first registration accuracy measurement mark 201 will be referred to as incident light beam 204c, and a beam reflected on the upper surface of first registration accuracy measurement mark 201 will be referred to as reflected light beam 204d. A beam incident on an upper surface of the chip will be referred to as incident light beam 204i, and a beam reflected on the upper surface of the chip will be referred to as reflected light beam 204j. At this time, intensities of reflected light beams 204b, 204d and 204j are measured as shown in FIG. 13. Intensity of a light beam is higher when the light beam is reflected from a closer point. Intensities corresponding to reflected light beams 204b, 204d and 204j are represented by light intensities 219, 220, and 223, respectively. In order to detect the step by referring to the intensities, a step between light intensities such as observed at a boundary 224 or 227 is detected. Thus, the position of the edge of the registration accuracy measurement mark is found, and hence amount of offset in registration is found.
However, recently, degree of integration of semiconductor devices such as ICs and LSIs has been ever increasing, and the buses have been rapidly miniaturized. Accordingly, in the method of improving registration accuracy, measurement with the precision of 0.1 .mu.m to 0.05 .mu.m has come to be required.
The process for manufacturing devices includes repetition of patterning of interconnections.elements, deposition of an insulating film and patterning of interconnections. Here, when an interconnection of an upper layer is to be formed while unevenness of the underlying layer is not sufficiently made planar, focusing fails in the step of lithography for patterning the interconnection, resulting in formation of a dull or blurred interconnection pattern.
Widths of interconnection of VLSIs has been narrower and narrower as the devices have been miniaturized to high degree, and accordingly, margin of depth of focus (D.O.F) in lithography has been decreasing. In other words, tolerable range in focusing at the time of lithography becomes smaller. Therefore, unless the device surface is almost perfectly planarized over wide range, satisfactory patterning of interconnections cannot be ensured.
As the margin of the depth of focus decreases, the step of the registration accuracy measurement mark is reduced by rapid strides, from the order of several hundreds to several tens A. As a result, the conventional method of finding a step in accordance with light intensities described above can no longer catch up to ensure sufficient accuracy. More specifically, when a step is very small, light intensities 220 and 223 are as shown in FIG. 13, making it difficult to clearly identify the boarder 227. Difficulty in identifying the step leads to difficulty in measurement or inspection of registration accuracy.